For wireless communication transmit diversity techniques are a known technology to mitigate the detrimental effects of fading. One class of transmit diversity technique use so-called space-time codes. Space-time codes typically require knowledge of channel coefficients at the receiver side which knowledge may be obtained from transmission channel estimation.
However, transmission channel estimation is a significant problem in multiple input multiple output MIMO wireless communication systems as a plurality of subchannels has to be estimated and the energy of pilot symbols has to be distributed over a plurality of transmit antennas.
In view of the above, differential transmit diversity schemes being based on unitary space-time modulation schemes have been proposed in B. Hochwald and W. Swelden: Differential Unitary Space-Time Modulation. IEEE Transactions on Communications, 48(12): 2041-2052, December 2000 and B. L. Hughes: Differential Space-Time Modulation. IEEE Transactions on Information Theory, 46(7):2567-2578, November 2000, where transmission symbols are used for set up of unitary transmission symbol matrices.
As another alternative, differential transmit diversity techniques requiring no channel estimation and being based on so-called orthogonal designs have been proposed for two transmit antennas in V. Tarokh and H. Jafarkhani: A Differential Detection Scheme for Transmit Diversity. IEEE Journal on Selected Areas in Communications, 18(7):1169-1174, July 2000, and further been generalized to more than two transmit antennas in H. Jafarkhani and V. Tarokh: Multiple Transmit Antenna Differential Detection from Generalized Orthogonal Designs. IEEE Transactions on Information Theory, 47(6):2626-2631, September 2001.
A disadvantage of differential space-time block codes from orthogonal designs is the limitation to unit length transmission symbol vectors. Here, distance properties being relevant for the achievable error rate are determined by distances between constellation elements of a phase shift keying PSK modulation scheme forming the basis for the setup of differential space-time block codes. It should be noted that distance properties of a M-ary phase shift keying PSK modulation scheme are only advantageous for M≦8.
In other words, for higher order modulation it would be preferable to code information, both, in phase and amplitude, e.g., through use of quadrature amplitude modulation QAM schemes. For differential modulation with respect to single transmit antenna systems there exist proposals for modulation schemes using multiple amplitude levels, e.g., in H. Rohling and V. Engels: Differential Amplitude Phase Shift Keying (DAPSK)—a New Modulation Method for DTCB. In International Broadcasting Convention, Pages 102-108, 1995, and F. Adachi and M. Sawahashi: Decision Feedback Differential Detection of Differentially Encoded 16 APSK signals. IEEE Transactions on Communications, 44, April 1996, pages 416-418. According to these proposals a signal space constellation is constructed from concentric phase shift keying PSK constellations. This is the reason for calling the method differential amplitude/phase shift keying DAPSK.
Further, there exits a proposal considering differential transmit diversity with multiple amplitude levels, see X.-G. Xia: Differentially En/decoded Orthogonal Space-Time Block Codes with APSK Signals. IEEE Communications Letters, 6(4):150-152, April 2002. It is proposed to select transmission symbols for input to the differential space-time modulator from an amplitude/phase shift keying APSK constellation, in particular a concentric phase shift keying PSK constellation having multiple phases and multiple levels of amplitudes. Therefore, this proposal only uses differential amplitude/phase shift keying DAPSK signals as proposed in H. Rohling and V. Engels: Differential Amplitude Phase Shift Keying DAPSK—a New Modulation Method for DTCB. In International Broadcasting Convention, Pages 102-108, 1995 as input for a unitary space-time modulator according to B. Hochwald and W. Swelden: Differential Unitary Space-Time Modulation. IEEE Transactions on Communications, 48(12): 2041-2052, December 2000.